searay your not necessarily wrong.. However every circumstance is different. Your right in the fact that when a segment is fused, you lose motion to that segment, and therefor, the adjacent segment will often suffer for this. Now those who were fused for lower back pain, and DDD, will likely suffer the worst result. (Disc Degeneration Disease). It has been clinically shown that fusion is less likely to help Discogenic pain, and if you spine has DDD, the levels above will obviously suffer when motion is taken out of one or two segements (fusion).
Fusion is much more successful in helping with Neurological symptoms... sciatica, shooting pain into feet, drop foot, urological symptoms , etc. Now what Im pointing out if there is a person with structural defects , leading to a slipped verabrea , (spondylolithesis), or one whom had a genitic defect such as a ''pars defect '', or scoliosis... they may fair much better from surgical intervention. If there is a specific injury to the lower lumbar, and the levels above are free of degeneration, the re visitation of surgery should be much less likely. this is at least what I have come to believe after many , many hours of research..months.
I had L4, L5, fused to S1 of the gate. At 31 y/o. medical techniques have evolved. as I mentioned most fusion were done via PLIFF approach which made a gutter to the side of your spine, and they literally had to lift a nerve up and move it up and out of the way. often leading to complications. Now they have neurological monitering, and have different less invasive approaches. Also graph techniques and hardware have improved dramatically.
I hope to at least make mid 50 's before having any issues in my L3.. if any at all during life.. thats the hope. neck is another story though